<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /* 这句话也是我们CSS的第一行代码 */
    * {
      margin: 0;
      padding: 0;
    }

    span {
      background-color: pink;
      margin: 20px;
    }
  </style>
</head>

<body>
  123
  <ul>
    <li>acccc</li>
  </ul>
  <span>行内元素尽量只设置左右的内外边距</span>
</body>

</html>